Getting AttributeError while using 'taskadmin ls'

[dmangla3]

OS : Ubuntu 16.04
Python : 3.6

AttributeError Traceback (most recent call last)
in ()
----> 1 Task.all()

/home/anonymous/anaconda3/lib/python3.6/site-packages/boto/sdb/db/model.py in all(cls, limit, next_token)
102 @classmethod
103 def all(cls, limit=None, next_token=None):
--> 104 return cls.find(limit=limit, next_token=next_token)
105
106 @classmethod

/home/anonymous/anaconda3/lib/python3.6/site-packages/boto/sdb/db/model.py in find(cls, limit, next_token, **params)
95 @classmethod
96 def find(cls, limit=None, next_token=None, **params):
---> 97 q = Query(cls, limit=limit, next_token=next_token)
98 for key, value in params.items():
99 q.filter('%s =' % key, value)

/home/anonymous/anaconda3/lib/python3.6/site-packages/boto/sdb/db/query.py in init(self, model_class, limit, next_token, manager)
30 self.manager = manager
31 else:
---> 32 self.manager = self.model_class._manager
33 self.filters = []
34 self.select = None

AttributeError: type object 'Task' has no attribute '_manager'